1. Field of the Invention
The present invention relates to a nonaqueous electrolyte which suppresses the decomposition of a solvent and a nonaqueous electrolyte battery whose cyclic characteristics are improved by employing such a nonaqueous electrolyte.
2. Description of the Related Art
In recent years, electronic devices such as portable telephones and notebook type personal computers have been progressively cordless and portable and thin, compact and light-weight portable electronic devices have been successively developed. In addition, a variety of electronic devices results in the increase of the consumed electric power. Accordingly, it has been increasingly demanded for batteries, especially, secondary batteries serving as the energy sources of these electronic devices to have high capacities.
As the secondary batteries which have been hitherto utilized, there are exemplified lead-acid batteries, nickel-cadmium batteries and nickel-metal hydride batteries. Further, as the secondary batteries having high capacities, lithium-ion batteries have been put to practical use. However, the conventional lithium-ion batteries can not undesirably sufficiently satisfy the demands of mobile devices or the like.
Further, as the portable devices come into wide use of common consumers, the devices have been frequently left under high temperature such as the inside of vehicles in the daytime. Thus, it has been demanded for the portable devices to give any danger to users in the above-described case as well as to be employed without any problems after that.
Now, the structure of the above-described lithium-ion secondary battery will be described below. On a cathode current collector made of a thin aluminum foil, a cathode active material composed of, for instance, LiCoO2 and graphite is laminated. On an anode current collector made of a thin copper foil, an anode active material composed of carbon, coke, graphite, etc. is laminated. These members respectively form electrodes. A separator as a thin film made of polypropylene, polyethylene or the like and having pores is sandwiched in between the electrodes to obtain a battery body. The obtained battery body is sealed in a cylindrical or a prismatic battery can made of nickel plated iron, stainless steel, nickel plated stainless steel, aluminum or aluminum alloy, nickel, copper, titanium, etc. and used.
Further, a lithium polymer secondary battery has a sandwich structure that a part between the electrodes is filled with a gel electrolyte obtained by swelling polymers such as polyacrylonitrile (PAN), polyethylene oxide (PEO), polyvinylidene fluoride (PVDF) with electrolyte solution including lithium salts. Also in this case, a separator made of polyethylene or polypropylene may be employed. Such a unit cell having a sandwich structure is packaged by a sealing material composed of a metallic thin film such as an aluminum foil and a plastic film such as nylon, polyethylene, polypropylene, polyethylene terephthalate, etc, as a sealing container.
In order to obtain a battery having a high energy density and excellent in its large current discharging characteristics, low temperature characteristics and cyclic characteristics, a chemical and electrochemical stability and a high ionic conductivity are required for an electrolyte.
Since there is not a solvent which satisfies all demands and performances by itself in the lithium ion battery, a solvent obtained by mixing ethylene carbonate with chain carbonate as disclosed in Japanese Patent Application Laid-Open No. hei 4-162370 is employed. However, since the steam pressure of the chain carbonate is high, for instance, when an aluminum laminate film or the like is employed as an outer packaging material, the solvent of the battery swells under high temperature, which may be possibly generated when a mobile device using the battery containing the solvent is left in a vehicle in the daytime, so that the circuit of the mobile device is inconveniently overwhelmed. Further, the high steam pressure is considered to be a factor for lowering a reliability. For attempting to lower the steam pressure, there have been proposed a method for making an electrolyte gel (see Japanese Patent Application Laid-Open No. hei 5-109310 or the like) or a method for using a halogenated solvent (see Japanese Patent Application Laid-Open No. hei 11-102727 or the like).
However, although the electrolyte gelled, in case many low boiling point solvents were included, the steam pressure was not effectively lowered. Further, in the conventional halogenated solvent, as an electrochemical stability was lowered, there undesirably arose the deterioration of cyclic characteristics which might be generated as a result of a continuous decomposition of the halogenated solvent due to repeated charging and discharging operations.